iOS - 淡入淡出切换rootViewController

1、UIModalPresentationStyle（弹出风格）

typedef enum {
    UIModalPresentationFullScreen = 0,
    UIModalPresentationPageSheet,
    UIModalPresentationFormSheet,
    UIModalPresentationCurrentContext,
} UIModalPresentationStyle;

这四种方式在iPad上面统统有效，但在iPhone和iPod touch上面系统始终以UIModalPresentationFullScreen模式显示。

UIModalPresentationFullScreen代表弹出VC时，VC充满全屏;
UIModalPresentationPageSheet代表弹出VC时，VC的高度和当前屏幕高度相同，宽度和竖屏模式下屏幕宽度相同，剩余未覆盖区域将会变暗并阻止用户点击.这种弹出模式下，竖屏时跟UIModalPresentationFullScreen的效果一样，横屏时候两边则会留下变暗的区域；
UIModalPresentationFormSheet这种模式下，VC的高度和宽度均会小于屏幕尺寸，VC居中显示，四周留下变暗区域；
UIModalPresentationCurrentContext这种模式下，VC的弹出方式和弹出VC的VC的父VC的方式相同。

2、UIModalTransitionStyle（弹出时的动画风格）

typedef enum {
        UIModalTransitionStyleCoverVertical = 0,
        UIModalTransitionStyleFlipHorizontal,
        UIModalTransitionStyleCrossDissolve,
        UIModalTransitionStylePartialCurl,
} UIModalTransitionStyle;

从底部滑入，水平翻转进入，交叉溶解以及翻页这四种风格可选。这四种风格不管是iPhone还是iPad都会根据我们指定的风格显示转场效果。

3、例子

// 登陆后淡入淡出更换rootViewController
- (void)restoreRootViewController:(UIViewController *)rootViewController
{
    typedef void (^Animation)(void);
    UIWindow* window = [UIApplication sharedApplication].keyWindow;
    
    rootViewController.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
    Animation animation = ^{
        BOOL oldState = [UIView areAnimationsEnabled];
        [UIView setAnimationsEnabled:NO];
        [UIApplication sharedApplication].keyWindow.rootViewController = rootViewController;
        [UIView setAnimationsEnabled:oldState];
    };
    
    [UIView transitionWithView:window
                      duration:0.5f
                       options:UIViewAnimationOptionTransitionCrossDissolve
                    animations:animation
                    completion:nil];
}